1. Field of the Invention
The present invention relates generally to a power supply apparatus for resistance welding, and more particularly to a power supply apparatus of a type using transistors for the control of a welding current.
2. Description of the Related Art
In order to provide a control of a welding current discharged from a capacitor for storing a welding energy, a transistor-type resistance welding power supply apparatus makes substantial use of only transistors intervening between the capacitor and welding electrodes without interposing a welding transformer therebetween. This system is considered to be advantageous for use in fine spot welding due to rapid build-up properties of the welding current and to an arbitrary provision of upslope or downslope waveform control.
Such a power supply apparatus ordinarily provides a feedback constant power control so as to allow the amount of welding heat to be kept at a certain value in spite of a possible change in the welding current or a voltage across welding electrodes during the weld time. For the provision of this feedback control, a shunt resistor is disposed in a power supply circuit to detect the welding current, with voltage sensor leads connected to the welding electrodes or their support members to detect a voltage between the welding electrodes so that the welding power is computed from the welding current detection value and the voltage detection value. Note that the transistor is in the form of a transistor group including several tens of transistors which are connected in parallel to one another, each transistor being a power transistor such as an FET or IGBT which withstands a large amount of current.
Such a conventional resistance welding power supply apparatus causes the transistor group to act as a kind of variable resistor between the capacitor and the welding electrodes so that provision is made of a control of the welding current so as to allow the welding power to coincide with the set value, for example, as described above. For this reason, there arises a problem that the transistor group may consume as the resistance heat in vain most of the energy discharged (fed) from the capacitor, resulting in a poor power supply efficiency. Furthermore, such a poor efficiency may prevent an acquisition of a large amount of current or elongated weld time.